Light sensitive tube



\ De. 17, 1935. GEFFCKEN ET AL 2,024,762

LIGHT SENSITIVE TUBE Filed March 14, 1953 mmmcu autumn HANS mug-wan KARL MULLER INVENTORS BY I 7 ATTORNEY Patented Dec. 1 7, 1935 UNITED STATES PATENT OFFICE LIGHT SENSITIVE TUBE Application March 14, 1933, Serial No. 660,640 in Germany March 14, 1932 5 Claims.

Our invention relates to improvements in photoelectric tubes and methods of making such devices, particularly those having electrodes consisting of a metallic base coated wth a layer of light sensitive material.

The underlying problem in the manufacture of photoelectric devices, as is known in the art, resides in the production of a light sensitive layer or film of high electron emitting properties. This fis accomplished by a decrease of the electron aflinity of the light sensitive layer or the work which must be performed in order to move electrons out through the surface of the layer into the surrounding space, and furthermore by an increase of the electron emission produced by incident light of given intensity.

Layers of this type have a very low electron amnity with correspondingly increased emitting properties. However, the emission current is still very low in the photoelectric devices of this type known in the art.

Accordingly, it is an object of the present invention to provide a photoelectric device preferably of the composite layer type in which the emission is increased by a multiple as compared with similar devices known in theprior art.

Photoelectric tubes frequently are filled with a suitable gas such as a noble gas resulting in a highly increased sensitivity and increase of the emission current. A major disadvantage, however, of gas filled photoelectric tubes consists in an early destruction of the sensitive cathode layer due to bombardment by positively charged particles produced by ionization of the gas.

Accordingly, it is a further object of the invention to provide a new light responsive electrode which is highly insensitive against bombardment by positive ions.

According to the invention, a metallic base serving as a support for the light sensitive layer is used which has been subjected to a chemical or mechanical roughening process before the light sensitive layer is applied. The novel character istics of a photoelectric electrode according to the invention may be generally described as involving a surface of the base or supporting metal of the light sensitive layer which, when looked at, is formed with relatively minute or microscopic projections or depressions as distinguished from an ordinary smooth surface as used in photoelectric devices known in the prior art.

By using an unsmooth or roughened surface of sponge-like or velvet-like characteristics, according to the invention as set forth, a substantially increased chemical activity is secured, resulting in highly increased photo-chemical properties of the light sensitive layer. Furthermore, a more substantial and uniform light sensitive layer is secured, uniformly covering the entire surface as compared with smooth light sensitive layers 5 according to the prior art in which the sensitivity is highly irregular on different points over the surface.

When applying a light sensitive layer to an unsmooth or roughened surface in accordance 10 with the invention, the layer will closely adhere to and follow the depressions or projections of the surface, resulting in an increased mechanical strength and uniform texture of the layer becoming less sensitive against mechanical shocks 15 and other disturbances tending to impair its photoelectrical properties as compared with a. similar layer applied to a substantially smooth surface. This obviously furthermore accounts for the highly increased favorable properties of 20 light responsive electrodes, according to the invention.

A further advantage of light responsive electrodes, according to the invention, consists in an additional decrease of the electron affinity, due 25 obviously-to the great uniformity and strength and uniform texture of the light sensitive layer obtained upon a rough or unsmooth surface.

The following practical example demonstrates the advantages and novel effects when using the 30 new method according to our invention in producing photoelectric devices as described.

According to the method of the present invention using a roughened or velvet-like silver base, a dark brown layer is obtained having a 35 cathode drop from to volts and an emission in a vacuum tube of 18 to 25 microamperes per lumen.

It has already been proposed to artificially increase the all-over surface of light sensitive electrodes by forming artificial projections. or depressions on the electrode surface. This procedure, however, was employed to afford a macroscopic increase of the all-over area and to produce point discharge effects, and is basically differentiated from the new method in accordance with the invention involving a roughening of the surface, resulting in more or less regular minute irregularities or depressions and projections, to produce an increase in chemical activity of the metallic base metal or intermediate layer, respectively.

A roughened unsmooth surface of the base metal, as proposed by the invention, may be produced mechanically by a sandblasting treatment 55 or by a shaking operation with a suitable material, preferably obtained as a precipitate in a liquid mixture. Alternatively, an electric method may be employed, such as a dusting-off process of metallic articles. Furthermore, a roughened or unsmooth surface may be produced by an etching process or by applying a rough coating to the metallic base. Another way consists in employing a chemical treatment consisting in first transforming the surface of the metallic base into a chemical compound, decomposing such compound into the metal, and easily removable preferably gaseous components. In this case the roughening process may be carried out simultaneously during the production of the base metal layer. The more velvet-like and chemically active the unsmooth or roughened surface in. ac-

' cordance with the invention, the greater will be the novel effects obtained.

Viewing the invention from another aspect, an explanation of the new increased photoelectricv effects may be accounted for by the particular space charge conditions prevailing in a device of the typeaccording to the invention. As may be understood, considerable space charges will be formed within the depressions of the roughened surface, resulting in an adherent space charge layer closely covering the entire light sensitive surface. Now, it is well known that by using thick oxide intermediate layers in composite tubes of the type under discussion, consid erably increased emission currents may be obtained, but the thus obtained layers are very easily and shortly destroyed by the ionic bombardment, thus making this type of cathode structure entirely unsuited for use in gas filled light sensitive, tubes. By using electrodes with a roughened base surface, however, the above disadvantage is completely eliminated, making it possible to provide intermediate oxide layers of substantial thickness, resulting in correspondingly increased electron emission as compared with a smooth base metal surface as used in the prior art. This effect is obviously due to a damping or cushioning action of the adherent space charge layer covering the light sensitive surface and serving as a protection therefor against impinging ionic particles.

The invention will be specifically described in connection with the accompanying drawings, wherein;

Figure l is a schemmatic View showing a photoelectric device embodying a cathode or light sensitive electrode as shown by Figure 1.

Figure 2 shows a greatly enlarged cross-section of a roughened cathode surface with a light sensitive layer applied thereon in accordance with the invention. Figure 3 illustrates in cross-section a photoelectric tube according to the invention provided with a filter for preventing the objectionable influence of short wave length radiation upon the cathode.

An example of producing a light sensitive or cathode electrode by a chemical method in accordance with the invention is carried out as follows: Referring to Figure 1 of the drawing; the light sensitive device comprising in the usual manner a glass bulb l4 including a wire-shaped anode IG and a. cathode sheet l5, preferably of silver, is connected to a high vacuum pumpfor evacuation. 'The electrodes [5 and [6 are shown to be connected to contact pins 18 and I9, respectively, mounted to a base I! of the tube [4. After the tube has'been evacuated, oxygen is introduced under low pressure and a glow discharge started between the electrodes in accordance with a wellknown process whereby the silver sheet I5 is covered with an oxide coating. Subsequently, the tube is heated close to its softening temperature in such a manner that the oxide coating on the 5 sheet I5 is again. decomposed, leaving a roughened or sponge-like surface on the silver electrode. Then the tube is processed and finished in the usual manner; that oxygen is again introduced, the electrode surface oxidized, the oxygen removed, and an alkali metal is introduced in vapor form and deposited to form a light sensitive film. Afterwards a noble gas may be introduced and the tube sealed up. The distinguishing and novel step resides in the first oxidizing process and subsequent reduction, resulting in the production of an unsmooth or sponge-like surface of the silver base metal.

Instead of producing an oxide with subsequent reduction, any other chemical compound may be 20 produced and subsequently-decomposed, such. as chloride, bromide, or sulphur compound, and the like, for obtaining a roughened surface of 'the base metal.

Experiments have shown that the photoelectric 25;. sensitivity in gas filled photoelectric tubes having an average of about 50 microamperes per lumen may be increased by using electrodes according to the invention to more than about microamperes per lumen.

The best gas filled tubes produced according to our invention have an output of 500-800 microamperes per lumen, a sensitivity of 300 microamperes per lumen being sufficient for most practical uses.

Furthermore, we have discovered that extremely sensitive photoelectric layers. constructedaccording to our invention are destroyed by blue, violet and ultraviolet rays. It appears that this effect is due to a photochemical decomposition 40; of the unstable and complicated chemical compounds forming a composite. photoelectric sensitive layer of this type. To avoid this drawback we provide an envelope non-permeable for such an objectionable radiation such as an envelope 4.5aof a brown-red colour. Such an envelope may consist of a colored glass bulb, used for manufacture of the tube itself or it may consist of a metal envelope having a coloured glass window; but we prefer to use normal glass bulbs for the produc- 50?. tion of the tubes, in which case the production of the light sensitive layer is carried out in a room illuminated by red light only. The bulbs are then coated with a brown-red cellulose-or other varnish absorbing all wave lengths shorter than 55:; about 5,500 A. U. or Angstrom units, that is the range comprising the lower portion of the visible spectrum (blue and violet rays) and the ultraviolet range below the visible spectrum (1 A. U.=l0 metre).

By these means the sensitivity of the tube is only reduced about 15-20%, but is remaining constant over an extremely long period.

Byusing the sensitivity destroying efi'ect of short wave length rays we have been able to manufacture photo-electric bulbs of exactly prescribed sensitivity. For this purpose the bulbs are made with an extremely high sensitiveness and before laquering, are exposed to the rays of a quartz-mercury or a metal salt arc lamp- Preferably the tubes are placed during this process in a chamber having two windows alternately opening, the first of which is arranged opposite to a standard carbon incandescent lamp, the II other being opposite said arc lamp. By alternatingly opening and closing said windows that is alternatingly measuring the output current produced by a standard illumination and desensitizing the photoelectric layer, an exactly predetermined value of sensitivity is easily attained. The tube is afterwards lacquer coated in the described manner, and by these means the attained sensitivity is maintained for an extremely long period. Further corrections may be made by suitably choosing the thickness of the varnish coating.

Referring to Figure 3 numeral 20 represents a glass bulb containing the light sensitive layer 2| deposited upon the inner wall of the bulb in the usual manner, and an anode in wire form shown at 22. The line 23 represents the lacquer film,used according to our invention and retaining objectionable rays such as blue, violet and ultra-violet rays liable to destroy the sensitive layer 2 I.

Referring again to Figure 2, the dimension of which is greatly exaggerated as regards the thickness of the layers, l represents the base metal such as a silver sheet as described, upon which there are formed projections ll of minute dimension to secure a velvet-like or sponge-like surface. The surface is then covered with an intermediate layer l3, such as an oxide of the base metal as described, and the intermediate layer coated in turn with a light sensitive layer I2 as shown.

Besides the described and illustrated modification, the invention, of course, is capable of other variations and modifications within its scope, and may be embodied in other styles or types of photoelectric devices. The photoelectric layer may be of any suitable composition and produced by any desired process applied either directly to the base metal or in conjunction with an intermediate layer as described.

We claim:

1. The method of adjusting the emission current during the manufacture of photo-electric tubes consisting in subjecting the light sensitive electrode to the influence of short wave length radiation of the lower visible and the ultra-violet range of definite intensity and over a definite 5 period.

2. A photoelectric tube comprising a light sensitive negative electrode of the composite layer type and a cooperating positive electrode included in a vessel and a light filter for preventing radiation of the lower visible and of the ultra violet range from effecting the light sensitive electrode.

3. A photoelectric tube comprising a cathode electrode having a highly light sensitive surface of the composite layer type and a cooperating positive electrode included in a vessel and a light filter disposed closely to said. tube and adapted to prevent short wave length radiation of the lower visible and of the ultra-violet range from effecting said light sensitive surface.

4. A method of adjusting the emission current of photo-electric tubes having a composite layer type light sensitive emissive surface consisting in subjecting the light sensitive surface to the influence of ultra-violet radiation of predetermined g5 intensity until the desired emission is obtained.

5. A photoelectric tube comprising a vessel, a negative electrode therein having a composite light sensitive layer, a co-operating positive electrode in said vessel, said vessel being provided with a window for passing controlling light rays to said cathode and a coating of red-brown coloring lacquer applied to said window for preventing ultra-violet rays and rays of the lower part of the visible spectrum from effecting the said light sensitive layer.

HEINRICH GEFFCKEN.

HANS RICHTER.

KARL MGLLER. o 

